The purpose of this proposal is to test the hypothesis that neonatal BCG vaccination leads to activation of immune response that may increase the susceptibility to HIV infection in breast-fed infants of HIV-positive mothers. This investigation stems from the candidate's research as a post-doctoral fellow in heterologous ^ immunity and will allow her to develop expertise in patient-based immunology research. Furthermore, it provides a long-sought opportunity to develop skills in international research and foster an academic collaboration with Latin America, the candidate's region of origin. The specific aim of this proposal is to evaluate the effect of BCG on expression of co-receptors required for HIV entry (CCR5 and CXCR4) and on susceptibility of mononuclear cells to HIV infection. The initial phase of the study will be performed in vitro, using cord blood cells from healthy infants. Determination of co- receptors levels will be performed using FACS analysis and single-round HIV infection will be measured with a recombinant GFP-expressing HIV-1. The clinical phase of the study will proceed after optimization of all assays involved, and will involve healthy neonates. This phase is a longitudinal assessment of co-receptor expression and HIV susceptibility of mononuclear cells before and after BCG vaccination. Activation markers and HIV susceptibility will be measured in cord blood mononuclear cells (pre-BCG) and peripheral blood mononuclear cells obtained from the same infants 6 weeks'after BCG vaccination. A significant proportion of infants infected with HIV through maternal-to-child-transmission (MTCT) are infected in the post-partum period through exposure to breastmilk. Although exclusive replacement feeding eliminates the risk of HIV acquisition through lactation, this option is often not safe or feasible for women in lower income settings due to stigma, high cost of formula, or poor access to potable water. The results from this study will provide a conceptual immunologic basis for further exploring the clinical effect of BCG-induced immune activation on host susceptibility to HIV. The effect of BCG on co-receptors required for HIV entry may have implications regarding the timing of BCG vaccination in HIV-exposed breastfed neonates.